1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to an apparatus for block encoding of an input image signal.
2. Related Background Art
In the field of such apparatus, there has already been known a digital VTR (video tape recorder) for digitizing image signals and effecting the recording and reproduction there of after block encoding.
In such a digital VTR, the correlation of the fields within a frame is usually discriminated at the compression and encoding of the digital image signal, and a frame process and a field process are switched according to the correlation, in order to reduce the amount of code at the encoding by a process of higher correlation, thereby controlling the code amount so as to conform to the code amount defined by the format of each digital VTR.
In such a digital VTR, the dubbing operation in digital signal form from another equipment such as a VTR is usually conducted, as shown in FIG. 1, by reproducing the image signal in digital format from a digital VTR 1, and recording said signal in a digital VTR 2. Also in such a dubbing operation of the image signal already recorded and reproduced in a digital VTR, the frame and field processes are switched by discrimination of the correlation of the fields, as in the first recording.
Such a dubbing operation between the digital VTR's can be most simply achieved with digital data, but, for the protection of copyright, there is conceived a method of avoiding the digital interface and conducting the dubbing after the digital data are returned to analog form.
Such analog connection provides an additional advantage of compatibility with conventional analog equipment, such as a title/illustration inserting equipment or an effecter 3 for achieving a wiping/fading effect, as shown in FIG. 2.
However, in the aforementioned dubbing between the digital VTR's, the switching of the frame/field process at the digital recording of the image signal is conducted in the recording VTR, based on the correlation of the fields, so that the re-encoding process at the recording VTR may become different from the initial encoding process (prior to dubbing). As a result, there may occur significant deterioration of the image.
More specifically, in such a dubbing operation, because of transmission errors in the analog transmission channel, representative values of the quantization at the re-encoding in the recording VTR may be different from those in the initial or preceding quantization. As a result, the representative values of quantization are aberrated from the original values as the dubbing operation is repeated, thus eventually resulting in significant deterioration of the image quality.
Also in the dubbing of the signal reproduced from an analog VTR, the formation of the optimum pixel blocks may be hindered by the loss of image correlation, resulting from variable time-base components, such as jitter (principally caused by mechanical reasons), generally contained in the reproduced signal.